Airports are a source of economic prosperity, but the main airports in many cities are reaching the limits of their capacity. Faced with the difficulty of expanding them, the development of multi-airport systems may be a mechanism for adapting to the pressure of demand. However, multi-airport systems are difficult to develop, as there is no clear typology and they depend on local particulars. On the other hand, and perhaps more relevantly, airlines have few incentives to operate at secondary airports. In a liberalized market, airlines will always tend to concentrate their activity on main airports. In this article, we present these basic principles of functioning that make multi-airport systems a difficult balance, although an interesting one to try to achieve.